1. Field of the Invention
The present invention relates to an oscillation device including a crystal oscillation circuit, and more particularly, it relates to a constant voltage circuit which enables a low consumption current of a crystal oscillation circuit.
2. Description of the Related Art
A conventional oscillation device is constituted of a constant voltage circuit which generates a constant voltage, and a crystal oscillation circuit which oscillates a crystal oscillator by the generated constant voltage. Such an oscillation device is broadly used in a clock, a cellular phone, a personal computer terminal or the like, whereby it is requested to suppress a consumption current.
In the oscillation device, for the purpose of suppressing the consumption current, it is important to decrease a voltage for driving the crystal oscillation circuit as much as possible. On the other hand, the crystal oscillation circuit has an oscillation stop voltage determined by oscillation characteristics of the crystal oscillator, an oscillation inverter, a load capacity and the like. It is known that the oscillation stop voltage linearly drops at a predetermined rate with a temperature rise in a usual operation temperature range (e.g., −40° C. to 85° C.). Therefore, it is necessary to set the voltage output from the constant voltage circuit so as to be higher than the oscillation stop voltage in an operation ensuring temperature range.
Here, there is known a technology in which a voltage response of the constant voltage to a temperature change is set so as to be the same as the voltage response or slope of the oscillation stop voltage to the temperature change in the operation ensuring temperature range (e.g., see Patent Document 1). FIG. 7 is a view showing a conventional constant voltage circuit. To decrease the consumption current of the crystal oscillation circuit, if a difference between the voltage response of the constant voltage to the temperature change and the voltage response of the oscillation stop voltage to the temperature change is decreased, the consumption current of the constant voltage circuit conversely increases. Therefore, when the current of a PMOS transistor MP2 and the consumption current of the constant voltage circuit are optimized, the consumption current of the whole oscillation device can be decreased as much as possible at the constant voltage of the oscillation stop voltage or more in the operation ensuring temperature range.    [Patent Document 1] JP-A-2008-236629